Electrochemical treatment of copper for improving its bond strength

ABSTRACT

A process and apparatus for treating a metal foil to enhance its ability to be bonded to a substrate comprise immersing the metal foil in an electrolytic cell having an electrolyte bath solution containing copper and applying a current having regularly recurring pulses and preferably flowing in only one direction to the cell. The current causes a dendritic layer of copper to be deposited on at least one surface of the metal foil at a first current density and bonded thereto at a second current density. The process and apparatus are particularly suited for treating copper foil.

This application is a division of U.S. patent application Ser. No.460,630 filed Jan. 24, 1983, now U.S. Pat. No. 4,468,293, which in turnis a continuation-in-part of Ser. No. 355,053 filed Mar. 5, 1982 nowabandoned.

This invention relates to an improved process and apparatus for treatinga surface of a metallic sheet or foil to enhance its ability to adhereto a substrate material. More particularly, a surface of a copper sheetor foil is treated to improve its ability to adhere to a resinoussubstrate.

Printed circuits are widely used in various electronic devices such asradios, televisions, electronic computers, etc. In the production ofprinted circuits, it is desirable to use copper, preferably in a foilform, because of copper's high electrical conductivity. Furthermore,where copper foil is carefully made and contains minimal elementalimpurities, electrical conductivity is very uniform over the extent ofthe electrical connection between two points.

In producing printed circuits, it is a common practice to bond metalfoil to a substrate material, generally a synthetic polymer, with anadhesive and to subject the composite structure to an acid etchingtreatment to form the desired circuit. However, because the adhesionbetween the metal foil and the substrate is weak, considerable efforthas been directed in the past to treating the metal foil so as toincrease its bond strength with the substrate. As a result of suchefforts, a variety of treatments have been developed which result in theformation of a roughened surface on at least one surface of the metalfoil. When the metal foil being used is a copper foil, these treatmentsgenerally comprise electrodepositing a dendritic copper layer on thesurface so that when coated with a hardenable plastic material, thetreated surface will form a tenacious bond, primarily a mechanical bond.

One type of prior art treatment for improving bond strength comprisesprocesses having multiple electrodeposition steps. One step of theseprocesses generally comprises applying by electrodeposition a nodularpowdery copper layer, primarily copper or copper oxide particles, on asurface of a metal foil. The particles form into random nodular clusterswhich increase the foil surface area. After applying the nodular layer,at least one overlocking layer of copper or other metal such as nickel,cobalt or chromium, which is not nodular in structure but which insteadconforms to the configuration of the first layer, is applied by a secondelectrodeposition step to provide a roughened surface while reducing thepowder transfer characteristics of the nodular copper layer. Thisoverlocking layer acts as an encapsulating coating to maintain theconfiguration of the surface projections intact. Typical of thesetreatments are those shown in U.S. Pat. Nos. 3,293,109 to Luce et al.,3,857,681 to Yates et al., 3,918,926 to Wolski et al., U.S. Re. Pat. No.30,180 to Wolski et al. and U.K. Patent Specification Nos. 1,211,494 and1,349,696 to Circuit Foil and Yates, respectively.

In some of the multi-electrodeposition step processes, anotherelectrochemical treatment is applied after the step of forming theencapsulating layer. In one such treatment, the electrical treatmentcomprises using a metallic ion containing electrolyte under conditionssuch as to electrolytically deposit a third microcrystalline layer ontothe surface being treated for further increasing its bond strength. Inyet another type of treatment, a metallic barrier is formedelectrolytically between the treated metal foil and the substratematerial. This metallic barrier is intended to prevent any interactionbetween the substrate and the underlying treated metal foil during thelaminating process. Within the prior art, it is known to form thismetallic layer from materials such as zinc, indium, nickel, tin, cobalt,brass, bronze, codeposited tin and zinc, chromium, aluminum, cadmium,cadmium alloys of tin, zinc or copper, and phosphorous-containingnickel. Typical of these types of treatments are those shown in US. Pat.Nos. 3,585,010 to Luce et al., 3,857,681 to Yates et al., 3,918,926 toWolski et al., 4,049,481 to Morisaki, 4,061,837 to Hutkin and 4,082,591to Morisaki et al., U.S. Re. Pat. No. 30,180 to Wolski et al., and U.K.Patent Application Nos. 2,073,778A and 2,073,779A, both to Furukawa.

Some of these prior art processes require that the various steps becarried out in either separate treatment tanks as part of a seriesoperation or in one tank with solution draining between the steps. Byhaving to use either a plurality of tanks or solution draining betweenthe steps, these processes tend to be inefficient and complex.

In some of the prior art processes, use of a variable current density issuggested. For example, in the Luce et al. (109) patent, a high currentdensity is used to deposit the dendritic layer. After the dendriticlayer has been deposited, a lower current density is used to apply theencapsulating layer. The surface being treated is exposed to the highcurrent density once and to the low current density once. In this sense,these prior art treatments are single cycle treatments.

It is also known in the prior art to form treated metal foil byelectrodepositing copper foil onto a rotating drum at a first currentdensity and thereafter applying a second current density to form adendritic structure on the foil while it is still on the drum. It isalso known that an overlocking layer could be formed on such a dendriticlayer by applying yet a third current density to the metal foil on thedrum. This type of approach for forming treated metal foil isillustrated in U.S. Pat. Nos. 3,674,656 to Yates and 3,799,847 toValdimirovna et al. and U.K. Pat. Nos. 1,543,301 and 1,548,550.

Although these multi-electrodeposition step treatments are capable ofproviding a foil with a dendritic outer surface, they do have thedrawback of requiring close control and regulation between the steps.Not only does each step need careful monitoring, but process variablesof each step, such as bath composition, current density, bathtemperature, etc. must be carefully coordinated with those of each otherstep. For example, if a two step process is used in which the bathposition is changed in the second step, close coordination is neededbetween bath composition and the other variables in the first step withthe new bath composition of the second step. These control andcoordination requirements do not yield a simple process. Even withcareful control of these processes, their complexity often gives rise toreliability problems. Furthermore, the multiplicity of electrodepositionsteps gives rise to the need for more space and equipment and theadditional expense associated with them.

In an attempt to simplify the overall process for treating copper foilto improve its bondability to a substrate, several processes have beendeveloped which utilize a single electrodeposition step to form adendritic layer on the copper foil. U.S. Pat. Nos. 3,220,897 to Conleyet al., 3,227,637 to DeHart, 3,322,656 to Dahringer et al., 3,328,275 toWaterbury, 3,454,376 to Luce et al., 3,518,168 to Byler et al.,3,699,018 to Carlson, and 4,010,005 to Morisaki and U.K. Pat No. 928,267to DuPont exemplify such processes. However, these processes frequentlyrequire additional treatments, bath agitation, and accurate control ofthe electrolyte bath composition, its temperature, and the currentdensity being used. For example, in the Dahringer patent, theelectrolytically formed layer is subsequently treated with a solutioncontaining a solute characterized as being capable of forming withcopper a compound which has low solubility in the solution. Two groupsof solutes have been found to be effective in achieving an improvementin bonding quality. The first group consists of compounds capable offorming a sulfide, telluride or selenide with copper. The second groupconsists of weakly acidic solutions of compounds capable of forming achromate, molybdate, tungstate or vanadate with copper.

In the Byler et al. patent, a cuprous cyanide bath is used toelectrodeposit copper dendrites on a surface of a clean copper sheet.However, the use of cyanide bath solutions is not desirable because oftoxicity and disposal problems.

Another approach for improving the bondability of copper to a substrateis to galvanically apply a coating of either cadmium or zinc to asurface of the copper. This approach is demonstrated in GermanAuslegeschrift No. 1,060,075 to Licentia. Yet another approach forimproving the bondability of copper foil uses an improved copperelectroplating method typically for use on a carrier material such asaluminum. The method comprises pretreatment of the carrier surface andelectrodeposition of copper foil utilizing an acidic plating bathcontaining copper, nitrate or fluoride ions which can be operated at asingle current density. This process is exemplified by U.S. Pat. No.4,169,018 to Berdan et al.

It is also known in the prior art to use different types of current waveforms for electrodeposition. Electroplating, by Lowenheim, McGraw-HillBook Co., 1978, pp. 160-163 discloses several forms of plating currents.One form is known as periodic reverse. Here, the direct current is madeto change direction at preset intervals so that for part of the cycleelectrodeposition occurs and for another part of the cycle deplatingoccurs. The more deplating that takes place as compared toelectrodeposition, the more sacrificial the cycle is said to be. Inorder to make this type of system worthwhile, the loss in overallefficiency caused by the reverse cycle must be compensated by someimprovement in the character of the deposit or of some other variable inthe system. A discussion of the various considerations involved in usinga periodic reverse current process to plate copper can be found in thearticle "Periodic Reverse Current Process in Electroplating Using AcidCopper Electrolytes", by J. Mann, Transactions of the Institute of MetalFinishing, Vol. 56, 1978, pp. 70-74.

Another form is known as pulse current plating. In pulse currentplating, the current is interrupted for certain periods of time. Duringthese time periods, the current density generally goes to zero. It isknown in the prior art to electrodeposit copper using the pulsed currentapproach. German Democratic Republic Patentschrift No. 134,785 toSkilandat et al., U.K. Pat No. 1,529,187 to Inoue-Japax Research, Inc.and the article "Electroplating with Current Pulses in the MicrosecondRange", by V. A. Lamb, Plating, August 1969, pp. 909-913 illustrate apulsed current approach for electrodepositing copper.

The utilization of pulsed current in the electroplating of metals hasnot yet met with any degree of commercial success primarily because ofthe relatively low amperage available at the plating cell. One suggestedapproach for overcoming this deficiency is to use an electronicswitching method for converting the flow of current from an AC linesource to periodic DC pulses to increase the peak amperages. In orderthat the rate of plating deposit may be increased, a conventional DCpower source supply may also be incorporated into the circuit. Such acircuit is illustrated in U.S. Pat. No. 3,959,088 to Sullivan.

German Democratic Republic Patentschrift No. 112,145 to Schmidt et al.illustrates a variation of the pulse plating technique for formingdendritic structures on metal foil. This variation comprises passingmetal foil through an electrolyte bath and subjecting the metal foil toa plurality of current pulses of relatively high current densitysuperimposed over a relatively low base current density. Thesuperimposed high current density pulses create a disturbed layer growththat leads to the development of nodular growth structures on the foil.Schmidt et al. suggest that the metal foil be subjected to 2-10 pulses.For each pulse, the foil is exposed to the high current density for atime period in the range of 0.1 to 10 seconds.

One of the deficiencies of the Schmidt et al. process is the productionof relatively long nodular or dendritic structures. These relativelylong dendritic structures can break off during the laminating processand become enclosed by the resin of the substrate. This latter problemis known in the prior art as a mechanical type of staining. Therelatively long dendritic structures produced by the Schmidt et al.process also have negative consequences with respect to etchability andresistance to abrasion. The dendritic structures produced by the Schmidtet al. process may extend far into the substrate material leading tolong etching times and to strong undercuts which reduce the dimensionalaccuracy of the etching.

The invention described herein embodies a single step electrochemicalprocess and apparatus for treating metal to enhance its ability toadhere to a substrate, particularly a non-metallic substrate. Metaltreated by the process of the instant invention exhibit superiorresistance to stress and wear, e.g. mechanical staining, and improvedpeel strength and powder transfer characteristics as a result of betterstructured dendrites being formed on the metal surface. In addition, theprocess described herein may be accomplished more rapidly, moreconveniently and with the consumption of less energy than prior artpractices.

In accordance with the invention described herein, an electrochemicalprocess and apparatus for treating copper sheet or foil to produce anadherent nodularized or dendritic surface structure which allows thesheet or foil to be securely bonded to a non-metallic substrate utilizesa multi-cycle fluctuating current to simultaneously form the dendriticsurface structure and bond it to the underlying copper sheet or foil ina single operation. The fluctuating current preferably flows in only onedirection and has regularly recurring pulses and a cathodic portion withfirst and second current densities each having a magnitude greater thanzero. Preferably, an uninterrupted current is utilized.

More particularly, an electrolytic bath comprising a coppersulfate-sulfuric acid solution is maintained in an electrochemical cell.The cell has an anode and a cathode. The cathode comprises the coppersheet or foil upon which the dendrites are to be deposited. Current isapplied across the cell either by a constant current source and afunction generator or a constant voltage source and a functiongenerator. The applied current preferably has a suitable wave form, suchas a square wave, a triangular wave, a sinusoidal wave, etc. The appliedcurrent causes clusters of copper particles to be deposited on andbonded to the copper sheet or foil. These clusters of copper particlesform the dendrites. They generally have a relatively fine structurewhich is highly desirable. It is believed that the relatively finedendritic structure is a result of the initiation of many nucleationsites during an initial current pulse and the renucleation of thedendritic structures each time there is another current pulse. Inaddition, undesirable columnar structures are avoided by not exposingthe dendritic structures to relatively long periods of time at currentdensities above the limiting current density. After the sheet or foilhas been treated in accordance with the instant invention, it may belaminated to a non-metallic substrate so as to form, for example, aprinted circuit laminate.

It has been found that by using the more efficient and simpler processand apparatus of the instant invention, a laminate having a peelstrength as great or greater than that typically produced by the priorart treatments can be formed. Peel strength is a conventionally usedterm to refer to the strength of the bond between the foil and thenon-metallic substrate.

Accordingly, it is an object of the present invention to provide animproved process and apparatus for treating metallic sheet or foil.

Another object of the present invention is to provide an improvedprocess and apparatus as above which enhances the ability of sheet orfoil to adhere to a non-metallic substrate.

Yet another object of the present invention is to provide an improvedprocess and apparatus as above which treats the sheet or foil morerapidly, more conveniently and with the consumption of less energy.

Yet another object of the present invention is to provide an improvedprocess and apparatus as above which uses a single electrochemical stepto form and bond a dendritic layer to a metal sheet or foil.

Yet another object of the present invention is to provide an improvedprocess and apparatus as above for forming a relatively fine dendriticstructure on metal sheet or foil and thereby impart improved peelstrength, wear resistance and resistance to mechanical staining to thesheet or foil.

These and other objects of the present invention will become moreapparent from the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an apparatus in accordance withthe instant invention.

FIG. 2 is an illustration of a current waveform which can be used by theinstant invention.

FIG. 3 is an illustration of an alternative current waveform which canbe used by the instant invention.

FIG. 4 is an illustration of yet another current waveform which can beused by the instant invention.

FIG. 5 is a schematic illustration of an alternative embodiment of anapparatus in accordance with the instant invention.

FIG. 6 is a graph showing the powder transfer characteristics of copperfoil treated in accordance with the instant invention as a function offrequency.

In accordance with this invention, a process and apparatus forelectrolytically treating a metal sheet or foil for enhancing itsability to adhere to a substrate, particularly a non-metallic substrate,are provided. Although applicable to many metals or alloys, theapparatus and process of the instant invention are particularly usefulfor treating sheet or foil made of copper and copper alloys. Laminateshaving high peel strengths, which are particularly suitable for printedcircuit applications, can be achieved through the use of the process andapparatus of the instant invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIG. 1, the apparatus of the instant inventioncomprises an electrolytic cell 10 having an anode 12, a cathode 14 andan electrolytic bath solution 16. The anode 12 and cathode 14 areconnected to a system 15 for applying a current having a desiredwaveform.

The cathode 14 comprises the sheet or foil upon which a dendritic layerof copper particles is to be deposited. The sheet or foil is very thinand preferably comprises printed circuit grade copper. The sheet or foilmay be held against a mechanical drum not shown in a manner well knownin the art and may be pulled through the solution by any conventionalpulling mechanism not shown as is known in the art. The mechanical drumassists in pulling the sheet or foil through the solution. The sheet orfoil is preferably electrically insulated from and not bonded to themechanical drum. In lieu of the mechanical drum and pulling mechanism,the sheet or foil may be attached to a temporary carrier such as a stripof aluminum, copper, iron, nickel, or any other suitable electricallyconductive metal and immersed in the solution 16 by any suitableconventional means not shown. Ultimately, the sheet or foil is removedfrom the carrier after the dendritic layer has been deposited. Iftreatment of two surfaces of the sheet or foil is desired, the sheet orfoil itself may be immersed in the bath solution without a supportstructure such as a carrier, a drum, etc.

Any suitable electrically conductive metal may be used for anode 12.Preferably, anode 12 comprises either lead or platinum or alloysthereof. Anode 12 is spaced a suitable distance from cathode 14.

The electrolytic bath solution 16 comprises a solution containingcopper. In a preferred embodiment, a copper sulfate-sulfuric acidsolution is used. The solution is preferably maintained eithersubstantially at room temperature or at a slightly elevated temperature.When maintained substantially at room temperature, the solutionpreferably has a concentration of copper, in the form of copper sulfate,from about 10 grams/liter, hereinafter g/l, to a saturationconcentration at room temperature of about 60 g/l. Below a copperconcentration of about 10 g/l, the current efficiency becomes too low toreasonably perform the process of the instant invention. Above thesaturation point, the copper sulfate precipitates and it becomessubstantially impossible to get more copper into the solution. In apreferred embodiment, the concentration of copper in the solution 16 atroom temperature is in a range of about 15 g/l to about 40 g/l.

The sulfuric acid in the solution 16 may have a concentration up to thatwhich causes copper to precipitate out as copper sulfate. In a preferredembodiment, the concentration of sulfuric acid is about 10 g/l to about100 g/l for a solution substantially at room temperature.

It should be recognized that the copper and sulfuric acid concentrationsare dependent upon the temperature of the bath solution. If desired,cell 10 may be provided with any well-known, conventional means notshown as are known in the art for maintaining the temperature of thebath solution at a desired level. The copper and sulfuric acidconcentrations discussed above may be adjusted if the solution 16 ismaintained at a temperature other than room temperature. At elevatedtemperatures, the concentration of copper could be proportionatelyhigher as the solubility limit increases with temperature.

The current applying system 15 preferably comprises a constant d-ccurrent source 18 and a function generator 20. The function generator 20provides the current applied to the cell 10 with a desired waveform. Thecurrent applied to the cell 10 is preferably an uninterrupted,multi-cycle, fluctuating current having a cathodic portion with firstand second current densities each with a magnitude greater than zero andflowing in only one direction. As shown in FIGS. 2-4, the appliedcurrent is a non-zero base cathodic current in which the second currentdensity is also the base current density. Any suitable current waveform,such as the square waveform shown in FIG. 2, the triangular waveformshown in FIG. 3, and the sinusoidal waveform shown in FIG. 4 may be usedas long as it has a cathodic portion with first and second currentdensities having a magnitude greater than zero. The constant currentsource 18 and the function generator 20 may comprise, respectively, anyconventional constant current source and function generator as are knownin the art.

The current applied across the cell 10 preferably has a cathodic portionwith a current density which has a first magnitude for a first period oftime t₁ and a second magnitude for a second period of time t₂ duringeach cycle of the current. The applied current preferably flows only inone direction so that only electrodeposition takes place during theelectrochemical treatment.

The current density required for producing a desired dendritic layer isdependent upon the concentration and the operating temperature of thebath solution 16. The first magnitude of the current density should beabove the magnitude of the limiting current density while the secondmagnitude of the current density is preferably below the magnitude ofthe limiting current density. The limiting current density may bedefined as the maximum current density at which the dischargeable metalspecies, in this case copper ions, is substantially depleted at thesurface of the metal foil or sheet. If the solution temperature israised, the current density used for the process of the instantinvention would have to be raised accordingly. If the copperconcentration were lowered or if the sulfuric acid concentration wereincreased, the required current density would be lower.

For a bath solution substantially at room temperature having the copperand sulfuric acid concentrations discussed above, the current has acathodic portion with a first current density having a magnitude ofabout 55 milliamperes/cm², hereinafter mA/cm², to about 350 mA/cm² and asecond current density having a magnitude of about 5 mA/cm² to about 50mA/cm². In a preferred embodiment, the current has a first currentdensity of about 150 mA/cm² to about 300 mA/cm² and a second currentdensity of about 10 mA/cm² to about 40 mA/cm². At the first currentdensity, copper particles from the solution are deposited on the surfaceof the cathode to form a dendritic layer. At the second current density,the dendrites are made to adhere to the surface of the copper foil orsheet. In performing the process of the instant invention, the dendritesare deposited upon the surface of the copper sheet or foil and made toadhere to the surface of the copper sheet or foil during a number ofcycles of the current.

By depositing a dendritic layer of copper particles on a surface of acopper sheet or foil to be laminated to a substrate, the surface becomesmore readily adherable to the substrate. This occurs because theparticles forming the dendritic layer are characterized by highlyirregular, knob-like projections which not only increase the exposedsurface area, thereby improving adhesion, but also enhance themechanical aspects of adhesion.

The current is preferably applied to the cell 10 at a desired frequencyand for a desired period of time known as the deposition time. Thefrequency of the current determines the number of pulses to which thecopper sheet or foil is subjected for a given period of time. Again, itshould be noted that both the frequency and the deposition time aredependent upon the copper and sulfuric acid concentrations in thesolution 16 and the temperature of the solution 16. The currentfrequency used should be sufficient to both form and bond the dendritesbut not so high that the applied current essentially becomes a straightline DC current. For the aforementioned copper sulfate-sulfuric acidsolution substantially at room temperature, the frequency is in therange of about 1 Hz to about 10,000 Hz, preferably from about 4 Hz toabout 1,000 Hz, and most preferably from about 12 Hz to about 300 Hz.

The deposition time, as well as being dependent upon solutionconcentration and temperature, is dependent upon the current densitymagnitude. The lower the current density, the more time it takes todeposit sufficient copper on the foil or sheet to form the desireddendritic structure. The deposition time should be greater than that atwhich not enough copper is deposited out but less than that at which toomuch copper deposits out and creates long dendrites susceptible to beingbroken off. For the aforementioned solution concentrations substantiallyat room temperature and the aforementioned ranges of current density,the deposition time should be from about 2 seconds to about 60 seconds,preferably from about 5 seconds to about 30 seconds.

In performing the process of the instant invention, it is desirable tosubject the sheet or foil to the first current density magnitude forrelatively short periods of time. The time period t₁ for which the foilis subjected to the first or higher current density should be less thanabout 0.125 seconds, preferably less than about 0.1 seconds. Forproducing treated metal foil having superior wear and stress resistance,peel strength, and powder transfer characteristics, there appears to bea criticality to the time t₁ at the first current density and thefrequency of the current. In a most preferred embodiment, the time t₁ atthe first current density should be less than about 0.04 seconds. Thenumber of cycles or pulses to which the sheet or foil is subjectedshould be greater than 10 pulses, e.g. 11 or more. Preferably, the sheetor foil is subjected to at least 25 pulses. By forming the dendriticstructures in this manner, undesirable columnar structures may beavoided and finer structures may be generated. It is believed that byusing this approach more nucleation sites occur initially and that thedendrites renucleate each time there is a pulse.

In lieu of the current applying system 15 of FIG. 1, it is possible tosuitably deposit a plurality of dendrites on a metal foil surfaceutilizing a voltage control system. A suitable voltage control system 25is shown in FIG. 5. The voltage control system 25 comprises a constantvoltage source 28 and a function generator 30. Function generator 30 isused to provide the voltage with a desired waveform. The voltagewaveform utilized should be capable of generating a current having awaveform with regularly recurring pulses, a desired frequency and acathodic portion with a first current density having a first magnitudegreater than zero and a second current density having a second magnitudeless than the first magnitude but greater than zero. The voltage andcurrent are applied to the cell 10' in the manner previously discussed.Cell 10' has an anode 12, a cathode 14 and a bath solution 16 such asthose described hereinbefore.

The voltage applied to the cell 10' may have any suitable waveform suchas a square waveform, a triangular waveform, a sinusoidal waveform, etc.Constant voltage source 28 and function generator 30 may comprise anyconventional constant voltage source and function generator as are knownin the art. The voltage control system 25 will obtain substantially thesame result as the current applying system 15.

The process for treating the metal sheet or foil to enhance its abilityto adhere to a substrate comprises placing the metal sheet or foil to betreated in an electrolytic cell containing an anode and an electrolyticbath solution containing copper. The sheet or foil comprises the cathodefor the electrolytic cell. An uninterrupted, multi-cycle, fluctuatingcurrent having a cathodic portion with first and second currentdensities having magnitudes greater than zero and a waveform withregularly recurring pulses is applied across the cell, preferably in asingle direction only. The fluctuating current is applied for a timesufficient to deposit copper from the bath solution onto at least onesurface of the copper sheet or foil. The deposited copper takes the formof a plurality of dendrites firmly bonded to the sheet or foil surface.Preferably, the dendrites have relatively large knob-like projections atone end. After the electrodeposition treatment has been completed, thetreated copper sheet or foil is removed and rinsed with a suitableliquid, e.g. water.

The treated sheet or foil may be laminated to a substrate using anywell-known bonding method such as a pressure and heat treatment. Thesubstrate to which the foil or sheet is to be bonded will vary dependingupon the use for which the laminate is intended and the serviceconditions under which such laminate will be used. Particularlyappropriate substrates which adapt the laminate for use in formingprinted circuits include epoxy-resin-impregnated FIBERGLAS supports,epoxy-impregnated paper, phenolic resin impregnated paper and the like.The substrate may also comprise both flexible and non-flexible supportssuch as polytetrafluoroethylene-impregnated FIBERGLAS,fluorocarbon-impregnated FIBERGLAS, MYLAR and the like. Other flexiblesubstrates include polyimides such as that known under the designationKAPTON and manufactured by DuPont.

If a pressure and heat treatment is used to bond the treated sheet orfoil to a substrate, the pressure and heat should cause the substratematerial or substrate coating to flow into the spaces formed by thedendrites and thereby improve the strength of the bond.

If desired, a layer of zinc, brass, nickel or any other suitablematerial may be formed over the dendrites to preclude problemsassociated with bonding copper to certain types of substrate material.Any well-known conventional apparatus and method for applying this layerto the dendritic layer may be used.

In order that the invention may be more fully understood, the followingexamples are given by way of illustration:

EXAMPLE I

An electrolytic bath solution containing 20 g/l of copper, as coppersulfate, and 45 g/l of sulfuric acid was prepared. A 2 oz/ft² wroughtC11000 copper foil was used as the cathode. The electrolytic cell wasprovided with a platinum anode spaced about one inch from the cathode. Afluctuating current having a rectangular waveform, such as that shown inFIG. 2, and a frequency of 1020 Hz was applied across the cell. Thefluctuating current had a cathodic portion with a first current densityof 200 mA/cm² for a first time period of 4.9×10⁻⁴ sec and a secondcurrent density of 25 mA/cm² for a second period of time equal to thefirst period of time. The deposition time was 20 seconds. Aftertreatment, the copper foil was removed from the cell and rinsed.

The foil was then laminated under heat and pressure to FIBERGLASimpregnated with epoxy resin following commonly accepted procedures toform a standard FR-4 rigid printed circuit laminate. The peel strengthof this laminate was measured according to the Institute forInterconnecting and Packaging Electronic Circuits Standard Test method2.4.8 and was found to be equal to 12 to 13 lb./in.

EXAMPLE II

Identical treatments to that disclosed in Example I were performedexcept that the fluctuating current had a frequency of 4 Hz. Peelstrength values of 8 to 10 lb./in. were obtained.

EXAMPLE III

Identical treatments to that disclosed in Example I were performed for adeposition time of 10 seconds at frequencies of 1020 Hz and 4 Hz. Peelstrengths of 8 and 6 lb./in., respectively, were obtained.

EXAMPLE IV

Triangular and sinusoidal waveforms, such as those shown in FIGS. 3 and4, were substituted for the rectangular waveform of Example I. Theremainder of the process was identical to the process of Example I. Forthese waveforms, the peel strength was generally lower than thatobtained in Examples I and II.

Accordingly, all of the above examples provide for a copper sheet orfoil that when laminated, such as for printed circuit applications, hasgood peel strength. A dendritic layer for improving the adherability ofthe sheet or foil to a substrate is applied to the copper sheet or foilusing a single step electrodeposition process which preferably isoperated with an uninterrupted, multi-cycle, fluctuating current havinga waveform with regularly recurring pulses and flowing in only onedirection. The process of the instant invention permits formation of thedendrites and bonding of the dendrites to the foil surface atsubstantially the same time. The process of the instant invention alsopermits the use of current densities lower than those generally used inthe prior art.

EXAMPLE V

To demonstrate the critical relationship between current frequency andthe powder transfer characteristics exhibited by copper foil treated inaccordance with the instant invention, a plurality of treated copperfoil samples were prepared as follows. An electrolytic bath solutioncontaining 20 g/l of copper, as copper sulfate, and 45 g/l of sulfuricacid was prepared. 2 oz/ft² wrought C11000 copper foil coupons were usedas cathodes. The electrolytic cell was provided with a platinum anodespaced about one inch from the cathode. A fluctuating current having awaveform such as that shown in FIG. 2 was applied across the cell. Thefluctuating current had a first current density of 200 mA/cm² and asecond current density of 25 mA/cm². The first and second currentdensities were applied for the same period of time. The current wasapplied at frequencies of 0.25, 1, 4, 16, 64, 256 and 1024 Hz for adeposition time of 15 seconds. After each treatment, the copper foilcoupon was removed from the cell, rinsed in water, dried, and weighed.

A piece of SCOTCH Magic Transparent Tape was pressed on each treatedcopper foil coupon and then peeled off by hand. A visual inspection ofeach tape was made to see if the transfer of metal took place.Thereafter, each treated copper foil coupon was weighed. If the weightchange was negative, it meant that part of the dendritic treatment hadbeen pulled from the foil. If the weight change was positive, it meantthat adhesive had been pulled off the tape.

The results of this test are shown in FIG. 6 where the weight change isplotted against current frequency. It can be seen from this figure thatgood results were obtained at frequencies in the range of 4 Hz to 1024Hz and surprisingly superior results were obtained at currentfrequencies in the critical range of 12 Hz to 300 Hz.

Prior to being subjected to the process of the instant invention, thesheet or foil may be subjected to any suitable cleaning treatment knownin the art. For example, it may be subjected to any type of electrolyticcleaning process, i.e. cathodic or anodic cleaning, and/or immersion ina sulfuric acid pickling solution.

While the instant invention has been described in terms of formingcopper dendrites on copper sheet or foil, it may be used in conjunctionwith other metals such as nickel, zinc or chromium.

While it is preferred to maintain the bath solution 16 eithersubstantially at room temperature or at a slightly elevated temperature,the process of the instant invention will also work at temperatures nearthe freezing temperature of the bath solution 16, e.g. about -80° C.Typically the process will be performed with the bath solution at atemperature in the range of about 15° C. to about 50° C.

While the examples illustrate waveforms having a cathodic portion with afirst current density for a first period of time and a second currentdensity for a second period of time where the two periods of time areequal, it is possible to use waveforms having one time period greaterthan the other.

While an uninterrupted multi-cycle, fluctuating current flowing in onlyone direction is preferably utilized, other currents such as either aninterrupted current or a periodic reverse current may be utilized.

While the invention has been described in terms of a particular coppersulfate-sulfuric acid electrolyte bath solution, the process of theinstant invention could be performed using other types of electrolytebath solutions.

While the instant process for treating copper sheet or foil has beendescribed in terms of specific examples, the desired surface treatmentcan be obtained via a wide combination of fluctuating current densities,frequencies and waveforms and the instant invention should not belimited to those specified herein.

The patents and publications set forth in this application are intendedto be incorporated by reference herein.

It is apparent that there has been provided in accordance with thisinvention an electrochemical treatment of copper for improving its bondstrength which fully satisfies the objects, means, and advantages setforth hereinbefore. While the invention has been described incombination with specific embodiments thereof, it is evident that manyalternatives, modifications, and variations will be apparent to thoseskilled in the art in light of the foregoing description. Accordingly,it is intended to enhance all such alternatives, modifications, andvariations as fall within the spirit and broad scope of the appendedclaims.

We claim:
 1. An apparatus for treating metal foil for enhancing itsability to adhere to a substrate, said apparatus comprising:means forcontaining an electrolytic bath solution containing a concentration ofmetal; an anode and a cathode immersed in said solution; said cathodecomprising said metal foil; means for providing a non-zero base cathodiccurrent having a desired frequency and a desired waveform comprisingrecurring pulses with each said pulse comprising a first portion havinga first current density for a first time period, a base portion having asecond current density for a second time period, said first currentdensity being greater than the limiting current density andsubstantially larger than said second current density, said secondcurrent density being less than the limiting current density, and saidfirst time period being less than about 0.125 seconds; said currentproviding means comprising a source of direct current and a functiongenerator for modifying the direct current signal from said source toproduce said current with said desired waveform; and means for applyingsaid current to said cathode and anode so that said foil is subjected tomore than 10 of said pulses so that metal from said solution isdeposited onto said foil as a fine dendritic plating having improvedpeel strength, improved resistance to wear and stress, and improvedpowder transfer characteristics.
 2. The apparatus of claim 1 whereinsaid source comprises a constant current source.
 3. The apparatus ofclaim 1 wherein said source comprises a constant voltage source and saidfunction generator generates a voltage having a desired voltage waveformand said current having said desired waveform.